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Derek Simmons used countless test subjects in his efforts to "clone" Ada Wong. The main impediment to those efforts was finding subjects with compatible genetic structure.

Using the extensive resources of The Family, Derek learned that one of his own researchers, a Carla Radames, would be a suitable candidate. Derek tricked Carla into taking part in the experiment because she believed Derek valued her and her work too much to risk her. She was wrong, and Derek succeeded in creating the clone of Ada he had desired.

While Carla looked and behaved similar to Ada, there was still a piece of her that remained Carla. Derek's relationship with Ada had only been professional, so he couldn't see that this clone was not the real Ada. Whether through misguided affections or a desire to revenge himself on Ada, he ignored any nagging doubt and began to train and treat Carla as if she were the real Ada Wong.

Carla, having lost most of her personality during the experiment, behaved as Derek wanted. But in the depths of her soul, there was still a spark of Carla's original self, and once it came to the surface, she vowed to destroy Derek, the man who had ruined her life.

Below is a note written by Carla just after she was transformed into Ada Wong.
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I am Simmons' loyal servant, Ada Wong. I will do anything he asks, and in return, he does whatever I ask. We are the perfect match and no one can come between us.

But recently, I can't help but feel some horrible emotion deep down every time I look into his eyes. I don't understand how I could feel this way when we are meant to be together. Some voice is screaming, "Kill him! Destroy his dreams! Don't let him get away with this," and it gets louder every day, but why? What's wrong with me? He loves me so much and I love him back. There's no reason for me to feel any of this. I can't imagine a world without him.

I've given everything to him. I would never want to destroy his dreams. Ever.

With his deep-seated delusion to replicating Ada Wong, Derek C. Simmons injected countless subjects with the C-Virus and repeated the experiment. However, a person with genes and a sequence close to Ada was indispensable in order to create the replicate. They weren't easily found, and the experiment went nowhere.
Meanwhile, in The Family's investigation, it was revealed that the genome sequence of Carla Radames, his trusted scientist, was the type idealized by Simmons. Simmons coaxed Carla into participating in the experiment. Carla, who adored Simmons, probably thought she would never be made into a test subject.
Then as Simmons expected, Carla was reborn as a perfect replication of Ada Wong.
Simmons was only able to have a business relationship with the original Ada and began to train her to behave heart and soul like Ada, piling on a perverted love and desire for subjugation as her instinct until Carla had apparently completely become Ada.
Before long, Carla Radames had lost most of her sense of self and was mentally and physically reborn as Ada Wong. However, her consciousness as Carla, which was only slight, had changed form into an intense hatred for Simmons, who had denied her entire existence.
A note of Carla's at the time of her rebirth as Ada Wong remains.
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"I am Simmons' faithful servant, Ada Wong.
An agent solely for Simmons, I pledge absolute loyalty to Simmons, I will do whatever he desires.
I hope he hears my wishes in return.
We are a first rate master-servant relationship. An ideal relationship no-one can interfere in.
But recently, dark feelings have been welling up in the depths of my heart whenever I see his eyes.
I don't know why.
The way I live with Simmons should be all there is.
Someone continues to cry to me in my head.
"Don't forgive him", "Kill him", "Crush all of Simmons' ideals"
The cry grows bigger day after day.
Why?
There isn't one reason to have any hatred toward Simmons, he loves me so much.
I love him, I can't even think of a world in which he isn't there.
I offer my entire body to Simmons.
It's impossible to harm the world he idealizes.
I can't"